Zea mays

The cell wall
Plant protoplasts are normally enclosed by a wall
(2.3, 2.4) which gives rigidity and protection to
the cdl but, unless impregnated with fany materials,
does nOt prevent water and solutes diffusing
across it to the plasmalemma. The walls of
adjacent cells are cemented together by a common
middle lamella (2.11, 2.12) so that plant cells are
immobile, although fibres and some other cells
elongate by tip growth and intrude ben"een the
neighbouring cells.
Following cell division the progeny usually
undergo vacuolation growth and theiT primary
walls also expand. Commonly in parenchymatOlls
tissue the middle lamellae break down at the sites
where several cells connect to each other and
intercellular spaces develop (2.59). When expansion
growth ceases some cell types undergo deposition
of a secondary wall (2.7 to 2.10). The
constitution of the secondary wall components
and (he orientation of its cellulose microfibrils
(2.60) is markedly differenr from that of the
primary wall.
Primary wall
Cytokinesis normally immediately follows mitosis
and so the two progeny are divided by a common
cell plate (2.25). The plate first appears at the
equator of the mitoric spindle (2.56) and then
advances centrifugally (2.56 to 2.58, 2.61) ro fuse
with the mother cell wall. The plare is formed
from fusing dicryosome vesicles (2.53) and is
delimited by a plasmalemma derived from the
dictyosome membranes (2.57, 2.58). Strands of
ER penetrate the plate and these form the central
dcsmotubular component of the plasmodesmata
(2.13,2.14) which connect the proroplasts of
adjacent cells. The unrhickened cell plate constitutes
the middle lamella common to both
daughter cells (2.57, 2.58). It is largely composed
of pectic substances and subsequently a thin
primary cell wall is deposited on both surfaces of
the plate (2.12, 2.15).
When a highly vacuolated cell divides, the
growing margins of the cell plate are marked by
dense cytoplasm (the phragmosome, 2.61) in
which dusters of shorr microtubules occur (1.56
to 2.58). In some tissues mitosis is not immediately
followed by cell plate development: in the
endosperm of many species the initially coenocytic
cytOplasm (2.5) later becomes divided by freelyforming
walls. These often develop in a tortuous
pauern and a similar phenomenon occurs in callus
tissue (1.62). In transfer cells tortuous invaginations
of the primary wall into the protoplast occur
(2.63) and such modifications of parenchyma cells
are common adjacem to vascular elements.
30
In thinner areas of the wall plasmodesmata are
often clustered together (2.2). with up to 60
present per square micrometer of wall surface, to
form pit fields (2.64). When secondary wall
deposition occurs, these regions often remain
unthickened and give rise to pits closed internally
by a pit membrane (primary wall, 2.10, 2.54,
2.65). Where lignification of the wall occurs, it
largely restricts the passage of water and nutrients
benveen protoplasts to the non-lignified pitted
regions (2.10).
In the tracheary elements of the protoxylem
(2.9) a lignified secondary wall is deposited
internal to the primary walls in discrete rings or a
spiral, but extensive tracts of non-lignified
primary wall lie between the thickenings. These
non-lignified regions are attacked by hydrolytic
enzymes released from the vacuole of the
degenerating protoplast, so that frequently only
the cellulosic skeleton ('holey' wall) remains in the
mature element to indicate the original position of
the primary wall (2.66).
The somewhat thickened walls of sieve tubes
(2.6). and the thick walls of many storage
parenchyma cells (2.54) are primary and do nOt
normally undergo lignification. During differemiation
of the sieve e1emems their protoplasrs
largely degenerate (2.67), leaving intact the
plasmalemma together with modified plastids
(2.68, 2.69), mitochondria, endoplasmic reticulum
and deposits of P-protein (2.67, 2.68). The end
walls of the sieve elements become modified as the
amorphous polysaccharide, callose, is deposited
within the wall around (he plasmodesmata (1.6,
2.67). The desmotubular component of the
plasmodesma disappears and eventually a wide
pore develops, ranging from 1-15 ).1m in diameter,
but this usually appears plugged in sectional
material (2.6, 2.68).
The primary wall contains up to 80% of irs
fresh weight as water, while the other componems
are predominantly polysaccharide. Biochemical
analysis shows thar cellulose constitutes 25-30%
by weight of the dried wall while hemicelluloses
constitute a further 15-25%, pectic substances up
(035%, and glycoproteins 5-10%.
Cellulose is a polymer of glucose with units
linked into long unbranched chains of up to
15,000 monomers. These are laterally hydrogenbonded
to form microfibrils (2.17, 2.70) several
micrometres long and 3.0-8.5 nm wide. The
microfibrils possess high tensile strength and
reinforce the wall in a form analogous to steel
rods in reinforced concrete. The other polysaccharide
components are non-fibrillar. It is
considered likely that the hemicelluloses are linked
to the microfibrils by hydrogen bonding. In turn
the hemicellulose is covalently attached to a